# Types of Waves

### After reading this section you will be able to do the following:

• Describe the different types of mechanical waves.
• Explain what electromagnetic waves are.

As previously discussed there are three primary categories of waves: mechanical, electromagnetic, and matter. Some more details on mechanical and electromagnetic waves will be discussed on this page. As a reminder, the big difference between mechanical waves and electromagnetic waves is that mechanical waves are disturbances that travel in a medium while electromagnetic waves do not require medium (they can travel in a vacuum!).

## Mechanical Waves

While mechanical waves cause a disturbance in a medium, they do not transport matter as they travel. They only transport energy. Within the category of mechanical waves there are 3 primary types: transverse,longitudinal, and surface

#### Transverse Waves

Imagine you have a long string. You tie one end to a tree and hold the other end so the string is under tension. You then move the end of the string you're holding up and down and you see this movement travel down the string towards the tree. In doing so, you're causing a displacement of the medium (the string) that is perpendicular/transverse to the direction of travel. In other words, you're causing a transverse wave!

#### Longitudinal Waves

Longitudinal waves also called compression waves and pressure waves. In longitudinal waves the displacement of the medium is in the same direction as the wave travels. To visualize this, imagine liquid in a tube with a plunger or piston at one end of the tube and a rigid wall at the other end. When you push on the piston you're causing displacement and pressure fluctuations in the liquid (the medium) that travel down the tube. Another way to visualize this is

#### Surface Waves

Surface waves travel along the interface between two different mediums. Some examples are ocean waves or seismic waves that often occur as a result of an earthquake. In surface waves the molecules of the medium undergo circular motion and are therefore neither transverse nor longitudinal. However, they can be described as having transverse and longitudinal components. Ground waves are an electromagnetic version of surface waves, but are beyond the scope of this material.

## Electromagnetic Waves

In the electricity and magnetism modules we discussed electric fields and magnetic fields, respectively. We also briefly discussed how a changing magnetic field can produce an electric field and vice versa. It is these linked changing electric and magnetic fields that form electromagnetic (EM) waves. As previously mentioned, these waves do not require a medium to travel. This means that while they can travel through a medium like air or a wall, they can also travel through a vacuum like outer space. Electromagnetic waves are used a lot in communications systems, such as those used to talk to satellites in space. It should be noted that electromagnetic waves and electromagnetic radiation refer to the same phenomenon.

One defining characteristic of an electromagnetic wave is its frequency, which is related to its wavelength and its energy. These concepts will be discussed more in a following page. The electromagnetic spectrum, which encompasses the full range of frequencies electromagnetic waves can have, is shown below.

### Review:

1. The three main types of mechanical waves are transverse, mechanical, and surface.
2. Mechanical waves only transport energy as they travel. They do not transport matter.
3. Electromagnetic waves are formed from linked changing electric and magnetic fields.